A sol-gel method for synthesizing a gel has the following advantages: Synthesis is performed at a low temperature compared to a fusion method and a mixing method in a vapor phase; Fluorine can be densely doped; A SiF group introduced as a result of doping removes a SiOH group, thereby improving light permeability in a vacuum ultraviolet region.
Because of these advantages, the sol-gel method has technically attracted attention as a new synthesis method of functional glass, ceramics and inorganic-organic complexes in the field of materials. However, the sol-gel method has the following drawback. In a process for drying a wet gel to obtain a dry gel, a gas/liquid interface (meniscus) is formed in a pore of the gel as a solvent is vaporized, with the result that capillarity is produced at the interface, contracting the gel. Cracks are therefore easily formed in the dried gel.
Water is used for preparing numerous wet gels as a polymerizing agent for a monomer and a solvent. Water has a large surface tension compared to other solvents of general use such as alcohols and has a high boiling point. Therefore, water is concentrated in a final stage of a drying process, producing large capillarity. For this reason, it is extremely difficult to dry a water-containing wet gel in a short time and to obtain a large dry gel. To obtain a highly reproducible dry gel while suppressing cracks, it is necessary to reduce capillarity, which is a cause of contraction stress, during a drying process, and to vaporize a solvent uniformly from the whole gel to contract the gel uniformly.
As methods for obtaining a crack-free dry gel, a chemical method and a physical method have conventionally been known in the art. In the chemical method, for example, a reagent is added in a process for producing a gel to modify e.g., the chemical structure of a gel skeleton. Examples of the methods known in the art include (1) a method in which a wet gel is prepared by adding a solvent having a high boiling point and a low surface tension as a drying control agent and capillarity is reduced in the final stage of drying (see, for example, T. Adachi et al. J. Mater. Sci. 4407, 22 (1987)), (2) a method in which a pore diameter of a wet gel is increased, in consideration of capillarity, which increases inversely in proportion to a pore diameter (for example, see H. Kotuku et al. Chem. Mater. 1, 398 (1989)), (3) a method in which micro particles are added to a wet gel, thereby increasing the pore diameter of the wet gel and improving the strength of the wet gel (see, for example, Japanese Patent Laid-Open Nos. 60-131834 and 64-87523), and (4) a method in which the chemical structure of a gel skeleton is modified to reduce capillarity while enhancing flexibility of the gel (see, for example, Japanese Patent Laid-Open No. 6-219726).
On the other hand, in the physical method, a crack-free dry gel is obtained without virtually modifying a wet gel. Examples of the physical methods known in the art include (5) a method in which a gel is extremely slowly dried to contract the gel uniformly, (6) a method in which a gel is dried under the temperature and the atmosphere strictly controlled (see, for example, Japanese Patent Laid-Open Nos. 6-219726, 2001-158615 and 2002-28472; U.S. Pat. Nos. 5,243,769 and 5,343,633; and F. Kirkaig et al. J. Sol-Gel Sci. Technol. 6, 203 (1996)), and (7) a method in which a gel is dried in a supercritical state, thereby eliminating the gas/liquid interface to prevent capillarity effect.
However, in the chemical methods, for example, the chemical composition and the production conditions of a gel are extremely limited. In addition, the reagents to be added may sometimes have toxicity such as carcinogenicity. Therefore, it is sometimes difficult to put the chemical method into a practical use. Also, the physical methods have problems. In the method (5), an extremely long time is required for preparing a dry gel. In the method (6), the solvent in a wet gel must be replaced once with a solvent easily dried and a special gas is required for drying. In the method (7), a pressure-proof container is required and the solvent of a wet gel must be replaced once with a solvent (CO2) easily performing supercritical drying. Therefore, the gel is limited in size by the size of the pressure-proof container.